This invention relates to improvements in laser apparatus. Lasers are devices which generate or amplify, and emit coherent electromagnetic radiation at higher frequencies than microwave frequencies, usually in the infrared and visible portions of the electromagnetic spectrum called optical radiation. The light emission from a laser is characterized by a narrow wavelength spread, i.e., it is essentially monochromatic, and by its spatial or in-phase relationship. Lasers may be fabricated from any active medium in which a population inversion may be established by suitable pumping. Typical of active laser media are neodymium glass, ruby, carbon dioxide, and helium-neon mixtures.
While the laser may take a variety of forms, the present invention is concerned with disc or slab type lasers which are characterized by having a thickness dimension along a line normal to the major faces thereof which is small as compared to the transverse dimension across the faces thereof.
Emission of coherent radiation from a laser requires population inversion, the condition which exists when a substantial number of the possible atomic or molecular radiating species in the active laser material are raised to a metastable energy state above the ground state of the species. When this condition exists, an incident photon of laser emission wavelength may stimulate a radiative transition from a metastable level to a lower level, which may or may not be the ground state of the species. Such radiative transitions are cumulative and self stimulating, resulting in the emission of radiation which is coherent and in-phase. Population inversion is achieved, for example, by irradiation of the laser medium with a high intensity of electromagnetic radiation at a wavelength of appropriate energy to raise the radiating specie to a metastable state when the radiation is absorbed thereby. Such inversion-causing radiation is referred to as pumping radiation, and the wavelength of the pumping of activating radiation is known as the pumping wavelength.
The activation or creation of a population inversion, essential for coherent electromagnetic radiation, is achieved by optical pumping, namely the irradiation of the active medium with optical radiation from a source usually referred to as a pump. The amount of inversion created within an active laser medium is a function of the flux density (energy per unit area) of the pumping optical radiation incident thereupon. It is therefore desirable that means be provided for maximizing the amount of pumping radiation from the pump which may be incident upon an active surface of the laser.
Neodymium doped glass (Nd:glass) has found frequent application in laser developments, and the pumping means conventionally used in such developments have been flashlamps. A flashlamp represents a long and narrow cylindrical radiation source which radiates into 4.pi. sterad, and reflector designs have been developed which provide optimum coupling of this pump source to disc or slab lasers.
Flashlamp pumped Nd:glass discs or slab lasers are not suitable for certain applications, for example as drivers in inertial confinement fusion systems, because of low conversion efficiency and limited repetition rate capability. Both limitations arise because of a very poor match between emission of flashlamp and absorption of Nd:glass and the low thermal conductivity of glass.
To avoid these problems, advanced developments in the solid state fusion driver art employ as sources of pumping radiation arrays or panels of light emitting diodes or excimer fluorescors or lasers. These devices provide a higher pump efficiency and reduce the amount of heat deposited in the glass laser slab or disc. Pumps of this type emit a generally collimated large cross section beam normal to a usually plane emitting surface, in contrast to the 4.pi. sterad radiation from long and narrow cylindrical light sources provided by flashlamps. However, the reflector designs which provide optimum coupling between flashlamps and lasers are optimized for the cylindrical, incoherent emitter, and are either unusable or strongly inadequate for use with the panel type pump sources.